It looks like a cross between a shark and a manta ray: In the Cretaceous period a bizarre fish with wing-like pectoral fins but otherwise a shark-like body “flew” through the sea, shows a fossil find. The shark apparently filtered microorganisms out of the water, explain the paleontologists. The results show an unexpectedly early evolutionary experimentation with “underwater flight” in plankton eaters. Because the eagle shark that has now been discovered was already spreading its “wings” 30 million years before the manta rays and their relatives, according to the dating of the fossil.
They roam the oceans with their mouths wide open: today’s representatives of these plankton-eating fish belong to the sharks and rays. These are only distantly related groups of the slab gill (Elasmobranchii). The plankton eater among the cartilaginous fish therefore also differ significantly in their physique: whale shark and basking shark have the elongated, torpedo-shaped shark body shape. Manta rays and Co (Mobulidae), on the other hand, have flattened bodies with wing-like pectoral fins. But the creature, which is now presented by an international team of paleontologists, seems to have characteristics of both groups and thus clearly stands out among the known species of shark and ray.
An amazing “hybrid creature”
The fossil was discovered in a quarry near the northeastern Mexican city of Vallecillo. The limestone there is very similar to the plate structures found in the southern Franconian Jura. Like these, they also often contain fossils. In the case of the finds in the Vallecillo limestone plates, it is a unique snapshot of the living environment of the open ocean in the Cretaceous period. The remains of sea turtles, sharks, marine dinosaurs and other aquatic animals have already been found. In the case of the current discovery, ammonite found found aids in dating. The bizarre fish slid through the water about 93 million years ago.
As the researchers report, closer examination of the fossil revealed the following: the cartilaginous fish had a body about 1.70 meters long with typical shark-like features – but there was one literally outstanding exception: the animal had wing-like pectoral fins with a wingspan of 1.90 Meters, which thus exceeded its hull length. Based on these characteristics, the paleontologists gave the fish the name Aquilolamna milarcae – eagle shark. In addition, his head had a noticeably blunt shape with a wide mau, which presumably once contained tiny teeth, the scientists report.
“Flying” on the hunt for tiny creatures
Based on these characteristics, they come to the conclusion: The eagle shark, with its long, narrow pectoral fins, glided through the Cretaceous ocean 93 million years ago – similar to today’s manta rays. “The physique of Aquilolamna milarcae is extraordinary. Because he could also – like other sharks – swim with blows of his forked tail fin. Manta rays cannot do this, ”explains co-author Eberhard Frey from the Natural History Museum in Karlsruhe. On the basis of this mode of locomotion and the characteristics of the mouth, the scientists assume: “It is very likely that the“ eagle shark ”was a plankton eater, similar to today’s whale sharks or manta rays,” says co-author Wolfgang Stinnesbeck from the University of Heidelberg.
According to the paleontologists, the extraordinary Cretaceous shark gives a new insight into the evolutionary history of the plate gill. They describe the previously unknown physique in sharks as an “unexpected evolutionary experiment with underwater flight”. Because wing-like pectoral fins in combination with a filtering way of life were previously only known from the manta rays and their relatives. However, these representatives of the plate gill appeared only 30 million years later in the history of the earth and, according to its characteristics, the eagle shark was apparently not an ancestor of these animals.
It is therefore obvious that the “underwater flight” with the pectoral fins in plankton-eating sharks and rays developed independently of one another in the course of their evolutionary history – a case of parallel evolution. The concept was a bit different, but the result was similar: the wing-like structures made it possible to glide smoothly through the underwater world while taking in plankton.
Source: CNRS, Jura Museum Eichstätt, specialist article: Science, doi: 10.1126 / science.abc1490